Calculation of x-ray-absorption spectra of strongly correlated systems

The $L_{2,3}$ edge x-ray-absorption spectra for transition-metal compounds are usually calculated retaining only those states in the ground-state wave function which correspond to a purely ionic configuration at the transition-metal site, as well as those arising from a single electron transfer from the surrounding ligand sites. We critically compare such a truncated basis approach to a complete basis approach, which is exact within the assumed cluster model, and discuss the limitations of the approximate method, specifically in the case of compounds with the transition-metal ion in a formal high oxidation state. It is shown that while the truncated basis approach may be adequate for divalent transition-metal compounds, it can lead to erroneous estimates for parameter strengths in the case of highly oxidized, and consequently, strongly covalent compounds.